A conventional air bag device is shown in FIG. 1 (see Japanese Patent Publications No. Hei. 4-365652, No. Hei. 5-139236, and the like).
This conventional air bag device M0 includes an air bag 1 formed as a folded bag, an inflator 2 for supplying gas for expansion of the folded air bag 1, an outer cover 3, and a bag holder 6 to which the air bag 1, inflator 2 and cover 3 are all secured.
The cover 3 comprises a top wall 4, including a predetermined breakable portion 4a formed in a substantially H-shaped configuration when viewed from the top surface of wall 4. Portion 4a will be broken when the air bag 1 is expanded. Cover 3 also includes four side walls 5 which are respectively disposed at right, left, front and rear positions around the periphery of the predetermined breakable portion 4a on the lower surface of top wall 4. Side walls 5 also extend substantially in a quadrangular prism shape. Each of the four side walls 5 includes a securing groove 5a on an inner peripheral surface thereof, while each of the front and rear side walls 5F and 5B includes an insertion groove 5d on the lower end face thereof.
The bag holder 6 comprises a bottom wall 7 for holding the air bag 1 and four vertical walls 8 which extend upwardly from the respective front, rear, right and left peripheral edges of the bottom wall 7. Walls 8 are also disposed on the inner peripheral side surfaces of the respective side walls 5. Each of the four vertical walls 8 includes at the upper end thereof a downwardly bent securing piece portion 8a on the outer peripheral side thereof.
Cover 3 can be held by the bag holder 6 in the following manner. The securing piece portions 8a of the respective vertical walls 8 are inserted into corresponding securing grooves 5a of the respective side walls 5. Then, brackets 9 are inserted into grooves 5d on the front and rear side walls 5F and 5B and thereafter fixed to the bag holder 6. The front and rear side walls 5F and 5B are held by the vertical walls 8F and 8B and brackets 9, so that the cover 3 can be held by the bag holder.
Also, an annular retainer 10, disposed within the air bag 1, includes a plurality of bolts (not shown) which respectively project downward. Accordingly, to assemble the air bag 1, inflator 2 and brackets 9 to the bag holder 6, these bolts may be respectively inserted into the air bag 1, inflator 2, bag holder bottom wall 7 and brackets 9, and then threadedly engaged with and fixed by nuts.
The reason why the two front and rear side walls 5F and 5B are selected as the portions to be held by use of the brackets 9 from among the four side walls 5 of cover 3 is that, when the air bag 1 is expanded and the top wall 4 opens, the front and rear door portions 4b and 4c of the top wall 4, enclosed by the substantially H-shaped predetermined breakable portion 4a, also open and the tensile forces thereof produced when the door portions 4b and 4c open act strongly in an outward or upward direction. This gives rise of the need to strongly retain the side walls 5F and 5B situated just below the front and rear door portions 4b and 4c.
In the conventional air bag device M0, the securing grooves 5a, to which the corresponding securing piece portions 8a of the bag holder 6 are to be secured, are all formed uniformly in the four side walls 5 of the cover 3 that extend in a substantially quadrangular prism shape.
The cross-sectional shape of every securing groove 5a, as shown in FIG. 2, is such that the height dimension h0 of the interior space 5b of the securing groove 5a is equal to the height dimension H0 of the securing piece portion 8a, while the width dimension h1 of the opening 5c thereof is smaller than the height dimension H0 of the securing piece portion 8a.
Therefore, in the conventional air bag device M0, it takes time and labor to insert and secure the securing piece portions 8a to their respective securing grooves 5a of the four side walls 5. This results in poor assembling of the cover 3 to the bag holder 6. Especially, for the purpose of reducing the cost of the cover 3, if the material of the cover 3 is changed from soft urethane to thermoplastic elastomer, such as polyolefin system material or the like, then the cover 3 is a little harder, which further lowers the ease of assembly of cover 3 to bag holder 6.